1. Field of the Invention
The present invention relates, generally, to methods and apparatuses for communicating voice and data in a wireless telecommunications network. More precisely, the invention is directed to forward link and reverse link diversity in a wireless telecommunications network.
2. Description of Related Art
Cellular telephones not only provide a means for emergency communications but also are rapidly becoming a primary form of communication in today's society. As cellular telephone usage becomes widespread, cellular telephone networks are becoming increasingly prevalent and are providing coverage over larger areas to meet consumer demands.
FIG. 1 depicts an example of a mobile station (MS) 101 operated by a mobile user that roves through a geographic area served by a wireless infrastructure including a first base station (BS) 121 with wireless sectors A 110 and B 111, and a second BS 122, with a sector C 112. In the course of such roving, MS 101 travels from position A 131 to position B 132 to position C 133, and will, as a matter of course, experience variations in signal strength and signal quality of the forward and reverse links associated with the base station or base stations that it is in contact with. Signal strength and signal quality can be especially undependable near the edges of the sectors, such as when MS 101 transitions from the area defined by the dotted line of Sector A 110 to the area defined by the dotted line of Sector B 111, or from Sector B 111 to Sector C 112. It is in these transition areas, as well as other areas of weak signal strength or poor signal quality, where dropped connections are likely to occur.
A dropped connection will occur even if only one of the forward and reverse links fails. This is due to a weakness in existing wireless systems. In existing systems, the forward and reverse link pairs originate from and are transmitted to the same base station or base stations in the case of soft handoff. FIG. 4 illustrates an exemplary link 22 between MS 24 and BS 26. Communications from BS 26 to MS 24 are called the forward link or downlink, and communications from MS 24 to BS 26 are called the reverse link or uplink.
The forward and reverse links utilize a number of forward and reverse channels. For example, BS 26 broadcasts on a plurality of forward channels. These forward channels may include, but are not limited to, one or more pilot channels, a sync channel, one or more paging channels, and multiple forward traffic channels. The pilot, sync, and paging channels are referred to as common channels. The common channel designation reflects that the channels are typically receivable by any and all mobile stations in the vicinity of BS 26. Generally, these common channels are not used to carry data (other than short message bursts for example), but rather are used to broadcast and deliver common and/or dedicated control information. Examples of common/overhead information include neighbor base station lists, access and monitoring parameters and configuration parameters. Examples of dedicated control information include pages, channel assignments and status requests.
The forward traffic channels are dedicated channels given that each forward traffic channel is intended for a specific mobile station and may carry user data. Typically, traffic channels convey both data and dedicated signaling such as handoff control messaging. Another type of traffic channel, called a dedicated control channel, may carry only signaling. Dedicated control channels are typically used in conjunction with a traffic channel or a supplemental channel (or data-only channel) such as a supplemental packet data channel that carries only data.
Forward traffic channels may also convey power control information for the reverse link that the mobile station is transmitting. Closed loop power control systems comprise measuring the received power level of the reverse link transmission as received at the base station and subsequently providing feedback in the form of up or down commands back to the mobile station via the forward link. These up or down commands may be in the form of information bits punctured onto the forward traffic channel. Other feedback information includes acknowledgements. Layer 2 acknowledgments for the receipt of individual signaling messages are typically embedded in the headers of signaling messages themselves on the opposite link.
As illustrated in FIG. 5, the forward traffic channels may contain data, control information and power control feedback for the reverse link. The data and control information are collectively identified as F in FIG. 5, while the power control feedback for the reverse link is identified as Rp. Similarly, the reverse link traffic channels may contain data and control information, which are referred to collectively as R in FIG. 5, and may contain power control feedback for the forward link referred to as Fp.
It should be noted that the power control feedback is typically implemented such that, in soft-handoff, even though the data and modulation of a traffic channel may be basically the same as transmitted from multiple base stations, the power control bits punctured onto the traffic channel may be different for each base station. Each base station punctures the traffic channel with the power control commands it deems appropriate. Thus, a mobile station may receive multiple contradicting power control commands. For this reason, standards typically specify that the mobile station follow specific rules. For example, a mobile station shall reduce its transmit power of the reverse link if any one base station requests the power to be reduced.
When the mobile station is transmitting to a base station on the reverse link, then it is also receiving a forward link from the same base station. For example, in FIG. 1, when MS 101 is at position A 131, it may be transmitting to and receiving from BS 121 in Sector A 110. If the reverse link to Sector A 110 fails, then the reverse link messaging that is contained in the reverse link will not reach BS 121. The messaging in the reverse link typically includes feedback information, such as acknowledgements, forward-link power-strength measurements or frame error indications, and this feedback, represented by Fp, may be used to control the power of the forward link. The reverse link may also contain messages requesting a new base station sector that is stronger than the current active sector(s). Since the feedback that the infrastructure depends on for proper link management is not available, the system is unable to recover from the reverse link failure and the call drops.
Similarly, if the forward link to Sector A 110 fails, the general procedures require that the MS 101 turn off the reverse link because it is no longer power controlled, i.e., MS 101 is no longer receiving Rp. These dependencies on each other are a weakness of the existing wireless systems. If one link fails, the entire connection fails. Furthermore, not only are the channels dependent upon one another because of signaling dependencies such as power control, but they also typically use the same physical link or medium, i.e., the same signal environment and path. Thus, there are two types of dependency: (1) control (such as feedback) and (2) link (same physical link).
Soft handoff, a state where a MS is in communication with two or more base stations simultaneously, improves the situation somewhat. Nevertheless, dropped connections can range from being a nuisance to devastating for cellular telephone users. For example, a dropped emergency 911 connection can be critical or even fatal. Dropped connections can create consumer frustration significant enough to cause the consumer to change service providers. Thus, the prevention of dropped connections is of major importance to cellular network providers.